Visualization of the catalyzed nuclear-spin conversion of molecular hydrogen using energy-selective neutron imaging

We present time-resolved neutron-imaging results on the ortho- to para-hydrogen conversion in the presence of a nanoparticle powder of the ferromagnetic catalyst gamma-Fe2O3. In particular, we were able to characterize the conversion rate as a function of time and position of molecular hydrogen with respect to the catalyst. Results are reported for liquid and solid molecular hydrogen at 15 and 10 K, respectively. We discuss how newly generated para-hydrogen poisons the catalyst, thus slowing the process and preventing the full conversion of large quantities of condensed molecular hydrogen, and we underline how the performance of the conversion critically depends on the loading procedure. Moreover, we suggest how a honeycomb distribution of the catalyst in a vessel can boost the conversion rates while minimzing the amount of material needed. Finally, we show how state-of-the-art energy-selective imaging using pulsed neutrons can be used to provide molecular specificity beyond what is currently possible for in situ and operando kinetic studies, with direct industrial applications such as the well-known "boil-off" problem associated with the low-temperature storage of molecular hydrogen.